1. Field of the Invention
The present invention relates to disk drives. More particularly, the present invention relates to a disk drive servo controller that utilizes an extended servo header.
2. Description of the Prior Art and Related Information
Computer systems often rely on disk drives for storing and retrieving data. Disk drives typically employ a moveable head actuator to frequently access large amounts of data stored on a disk. One example of a disk drive is a hard disk drive. A conventional hard disk drive has a head disk assembly (“HDA”) including at least one magnetic disk (“disk”), a spindle motor for rapidly rotating the disk, and a head stack assembly (“HSA”) that includes a head gimbal assembly (HGA) with a moveable transducer head for reading and writing data. The HSA forms part of a servo control system that positions the moveable transducer head over a particular track on the disk to read or write information from and to that track, respectively.
Typically, a conventional hard disk drive includes a disk having a plurality of concentric tracks. Each surface of each disk conventionally contains a plurality of concentric data tracks angularly divided into a plurality of data sectors. In addition, special servo information may be provided on each disk to determine the position of the moveable transducer head.
The most popular form of servo is called “embedded servo” wherein the servo information is written in a plurality of servo wedges that are angularly spaced from one another and are interspersed between data sectors around each track of each disk.
Each servo wedge typically includes a phase lock loop (PLL) field, a servo synch mark (SSM) field, a track identification (TKID), a wedge ID field having a binary encoded wedge ID number to identify the wedge, and a group of servo bursts (e.g. an alternating pattern of magnetic transitions) which the servo control system samples to align the moveable transducer head with or relative to a particular track.
Typically, the servo control system moves the transducer head toward a desired track during a course “seek” mode using the TKID field as a control input. However, in processing information, it is necessary to ensure consistency in the detection of bits composing a block of bits. One common approach directed to ensuring such consistency employs multiple stored fields including a phase lock loop (PLL) field to facilitate bit synchronization and a synch field to facilitate block synchronization. The synch mark field facilitates block synchronization by holding a special marker that is detected to “frame” data, i.e. to identify a boundary of a block. In contemporary hard disk drives, employing embedded servos, it is well known to provide framing of servo data via a servo synch mark (SSM) field. For example, in hard disk drives, a servo synchronization signal based on the moveable transducer head reading a servo synchronization mark (SSM) results in a read/write channel of the disk drive establishing a precise timing reference point for read/write operations.
Once the moveable transducer head is generally over the desired track, the servo control system uses the servo bursts to keep the moveable transducer head over that track in a fine “track follow” mode. During track following mode, the moveable transducer head repeatedly reads the wedge ID field of each successive servo wedge to obtain the binary encoded wedge ID number that identifies each wedge of the track. In this way, the servo control system continuously knows where the moveable transducer head is relative to the disk.
A feature referred to as disk synchronous write (DSW) is known. DSW is a feature that learns timing errors between servo wedges (e.g. SSM to SSM), and position error signals (PESs), and based on those learned timing errors, adapts the timing of the servo control signals (e.g. via frequency control) for use in the opening and closing of timing windows for the read/write channel for use in disk drive operations. For example, DSW may be employed in the reading of servo data and in the reading and writing of user data. By utilizing DSW, better efficiencies can be realized even in view of timing uncertainties due to disturbances such as eccentricity associated with the rotating disk.
In order to reduce timing uncertainties and to increase efficiencies, servo control systems may employ DSW. Particularly, DSW as currently employed, follows speed variations of the read servo data, and position error signals (PESs), and modifies timing control signals of the servo control system and of the read/write channel.
Eccentricity associated with a rotating disk drive, which results in associated eccentricity of read servo wedges upon which the servo control system operates, may occur for a number of reasons. When a disk is servo-written by a media writer and mounted in the HDA, it may not be properly aligned resulting in eccentricity associated with the rotating disk. Additionally, the disk drive may be dropped or be subjected to some sort of operational shock, which further adds to eccentricity problems. For all of these reasons, the disk may not be at the center of rotation resulting in the disk and the servo wedges turning with eccentricity.
When this occurs, the servo wedges appear to the servo control system as if they are not evenly spaced such that timing uncertainties are introduced thereby causing problems in both the reading and writing of data. Typically, the servo wedge timing of the eccentric disk is modulated by a once-around (i.e., a disturbance that occurs with every revolution). Accordingly, the eccentricity associated with the servo wedges results in timing uncertainties being introduced into the servo control system such that resultant servo control signals to control the writing of data also include these timing errors.
Unfortunately, present-day servo control systems before implementing DSW, when confronted with eccentricity problems, may often take a long time and be very inefficient in locking onto the servo header and implementing DSW, or may never lock onto the servo header resulting in the failure of the disk drive.